(1) Field of the Invention
The present invention relates to a novel sulfidoxymolybdenum dialkylphosphorodithioate compound which is valuable as a lubricant additive.
More particularly, the present invention relates to a molybdenum-containing lubricant additive which is excellent in the antioxidant property, the antiwear property, the friction-modification property and the heat resistance and has a reduced metal-corroding property.
(2) Description of the Prior Art
It is known that various compositions are effective as lubricant additives for engine oil and the like. However, the properties required for lubricant additives become recently more severe in view of resource-saving and energy-saving effects. ZDTP has been generally used for improving the antiwear property and the like, and sulfidoxymolybdenum dialkylphosphorodithioate compounds have also been used as proposed in Japanese Patent Publication No. 8426/65 or No. 27366/69 or Japanese Patent Application Laid-Open Specification No. 110796/81.
Known sulfidoxymolybdenum dialkylphosphorodithioate compounds have a primary alkyl group or a short-chain secondary alkyl group. A sulfidoxymolybdenum dialkylphosphorodithioate compound having a primary alkyl group has a high thermal decomposition temperature and is stable, and therefore, the compound is defective in that the antiwear property is poor at low temperatures.
On the other hand, a sulfidoxymolybdenum dialkylphosphorodithioate compound having a short-chain secondary alkyl group is defective in that since the oil solubility is low, it often happens that a concentration sufficient to exert a good lubricating effect is not obtained.
Moreover, each of the foregoing known compounds has a high metal-corroding property and is not suitable for practical applications (see Information to Users, published by Caterpillar Tractor Co. on Mar. 13, 1984).